1. Field of the Invention
The present invention relates to a process for elaboration of powders uniformly coated with ultrafine silicon-base particles.
2. Description of the Background
Elaboration of ultrafine particles of silicon, silicon oxide or silicon nitride or their alloys by gas phase chemical vapor deposition (CVD) using various means of energy supply, such as thermal, plasma or optical, is well documented. The particles produced thereby are significantly finer than those formed from other processes, i.e., having a typical average diameter of 0.01 .mu.m to 1 .mu.m as compared to 0.5 to 5 .mu.m, respectively. See P. Ho et al, J. Mater. Research, 4, 873 (1989) and L. M. Sheppard, Adv. Mater. Processes, 4, 53 (1987).
Such a gas phase process is advantageous because high purity and control stoichiometry is achieved. Generally, product purities can be obtained which correspond to the purities of the gas phase precursor, i.e., SiH.sub.4, SiH.sub.2 Cl.sub.2 and SiCl.sub.4, for example.
However, despite the advantages obtained using gas phase CVD to form particles of silicon, silicon oxide or silicon nitride, it remains very difficult to produce monosized particles thereby. Moreover, in using this method, it is difficult to avoid agglomeration of such ultrafine powders. Unfortunately, such drawbacks render difficult the application of the gas phase process to the synthesis of monodisperse and non-agglomerated powders.
Heterogeneous CVD growth of suspended particles is also rather well known, for example, EP 258027 discloses a process for producing polysilicon which entails contacting silicon particles in a fluidized bed with a silicon precursor gas at a decomposition temperature resulting in the deposition of silicon on the particles by CVD and deposition of silicon dust on the particle surfaces, and modifying the conditions so that a thin layer of silicon is deposited on the particle surfaces to bind the silicon dust.
Also, it is known that powders can be suspended in a fluidized bed. Further, suspension and growth of seed powders by gas phase CVD in a fluidized bed reactor is known for fabrication of ultra-pure silicon from silane (SiH.sub.4, Si.sub.2 H.sub.6), for example, for further processing into a silicon wafer.
For example, DE 3,910,328-A describes a fluidized bed reactor having heatable inside and outside walls, wherein the space between these walls is the reaction zone for the fluidized bed. The walls of the reactor are concentric cylinders with the diameter of the inner cylinder preferably being 10-95% of the diameter of the outer cylinder. Further, the walls are heated by induction or resistance heating or by conduction and the heatable area of the inner wall is 10-95% of the heatable area of the outer wall.
As another example, DE 3,910,343-A discloses a fluidized bed reactor consisting of a reaction vessel with an inner zone for the fluidized bed and a peripheral annular heating zone there around. The heating zone has a passage for transferring particles from the reaction zone to the heating zone at the top and a passage for transferring particles from the heating zone to the reaction zone at the bottom.